Lifts are used in a variety of different applications to raise and lower objects and people from a first elevation to at least a second elevation. In an industrial setting (e.g., a factory or warehouse), a lift may be used to transport heavy machinery and pallets of goods to and from balconies, mezzanines, basements, and/or between floors. Three types of lifts commonly used in an industrial setting are vertical reciprocating conveyors (VRCs), elevators, and scissor lifts.
A VRC typically includes a platform that supports the cargo and a pair of spaced apart vertical guide columns which guide the platform along a vertical path between the lower and upper levels. Fewer or more vertical guide columns may be utilized by the VRC (e.g., three or four vertical guide columns) depending on the application and type of cargo. Some VRCs employ a single mast from which the platform is cantilevered. To change the height of the platform, most VRCs employ an automated pulley that is mounted on a crossbar spanning the vertical guide columns and connected to the platform via a belt or chain. In general, safety regulations limit VRCs to carrying cargo and not passengers.
An elevator generally includes an enclosed car having a retractable door, a counterweight, a hoistway or shaft through which the car travels, a drive system, and various safety features that prevent free fall such as brakes and a governor. The safety features and design of an elevator make it suitable for human passengers, but the costs of installing and maintaining the elevator as well as other functional limitations may outweigh the benefit of human passengers in some industrial applications.
Scissor lifts employ a plurality of linked, folding supports arranged in a crisscross pattern that form one or more pantograph assemblies to operatively connect the platform to a base. The platform is raised by applying pressure to at least one of the folding supports in a manner that elongates the crisscross pattern and thereby propels the platform vertically. Descent is accomplished by collapsing the crisscross pattern. The crisscross pattern of folding supports is fairly resistant to sway and thus results in a relatively stable platform. As such, regulations typically allow an operator of a scissor lift to ride on the platform together with the cargo.
One common way to power a scissor lift is to provide a hydraulic actuator that exerts pressure on one of the folding supports to move the folding support into an upright position. The other folding supports, by virtue of their linked connection to the actuated folding support, are also turned upright, thereby causing the entire crisscross pattern of folding supports to elongate and push the platform in the upward direction.
A conventional scissor lift may depend solely on the hydraulic actuator to maintain the platform in a raised position. Because of the tendency of hydraulic actuators to slowly lose pressure over time, stationing the platform at an upper level for an extended period of time may result in the platform descending below the upper level. Unintentional descent of the platform may occur, for example, if heavy cargo is left on the platform for prolonged periods (e.g., overnight). Unintentional descent may also occur if a critical component of the scissor lift is accidentally removed during repair or maintenance while the platform is raised.
Another issue with conventional scissor lifts is that the platform may experience some sway during loading and unloading, particularly if the platform is loaded and unloaded at a high level where the crisscross pattern of folding supports is elongated to its maximum extent.